The present invention relates in general to the extraction of hydrocarbons from oil shale containing kerogen, and more particularly, to a method for pyrolytic extraction of shale oil from oil shale.
Oil shale is a fine grain sedimentary rock containing: (1) Organic matter derived chiefly from aquatic organisms or waxy spores or pollen grains, which is only slightly soluble in ordinary petroleum solvents, and of which a large proportion is distillable into synthetic petroleum, and (2) inorganic matter, which may contain other minerals. This term is applicable to any argillaceous, carbonate, or siliceous sedimentary rock which, through destructive distillation, will yield synthetic petroleum.
The hydrocarbon in oil shale is known as kerogen. Kerogen is a pyrobitumen, and oil is formed from kerogen by heating. It consists chiefly of low forms of plant life; chemically it is a complex mixture of large organic molecules, containing hydrogen, carbon, oxygen, nitrogen, and sulfur. Kerogen is the chief source of oil in oil shale.
The shale oil extraction process decomposes oil shale and converts kerogen in oil shale into petroleum-like synthetic crude oil. The process can be conducted by pyrolysis, hydrogenation, or thermal dissolution. The common extraction process (also known as retorting) is pyrolysis. In the pyrolysis process, oil shale is heated until its kerogen decomposes into vapors of a condensable shale oil and non-condensable combustible oil shale gas (shale gas can also refer to the gases that occur naturally in shales). In addition, oil shale processing produces spent shale, a solid residue. Depending on the technology, spent shale may include char, a carbonaceous residue formed from kerogen. Oil vapors and oil shale gas are separated from the spent oil shale and cooled, causing the shale oil to condense.
The temperature when perceptible decomposition of oil shale occurs depends on the time-scale of the process. In the above ground retorting process the perceptible decomposition occurs at about 300° C. (570° F.), but proceeds more rapidly and completely at higher temperatures. The rate of decomposition is the highest at a temperature of about 480° C. (900° F.) to about 520° C. (970° F.). The ratio of oil shale gas to shale oil depends on retorting temperature and as a rule increases by the rise of temperature. For the modern in-situ process, which might take several months of heating, decomposition may be conducted as low as 250° C. (480° F.).
Pyrolysis, being endothermic, requires an external source of energy. Most technologies use combustion of different fuels such as natural gas, oil, shale oil or coal, to generate heat, although some experimental extraction methods use electricity, radio frequency, microwaves, or reactive fluids for this purpose. Oil shale gas and char produced in the retorting process as by-products may be burned as an additional source of energy, and the heat of the spent oil shale and oil shale ash may be reused to pre-heat the raw oil shale. In addition to shale oil, other useful products could be generated during the process, including ammonia, sulfur, aromatic compounds, pitch, asphalt, and waxes.
The present invention provides a method heretofore unknown for the extraction of shale oil by pyrolytic decomposition of the oil shale into its hydrocarbon fractions.